1. Field of the Invention
The present invention relates to an apparatus and method of forming an image, and, more particularly, to an image forming apparatus which has a laser scanner unit for each color, and an image forming method for that apparatus.
2. Description of the Related Art
As shown in FIG. 1, a conventional image forming apparatus which uses a plurality of laser beams is designed in such a way that laser beams reflected at polygon mirrors 10 for individual colors (only one polygon mirror shown in FIG. 1) are irradiated on a photosensitive belt 11. As the polygon mirrors 10 turn, the top surface of the photosensitive belt 11 is scanned with the laser beams irradiated on the photosensitive belt 11, thereby forming a latent image thereon.
To form a distortion-free latent image on the photosensitive bolt 11, an image forming apparatus having a pixel-clock frequency correction means has been disclosed in Japanese Patent Unexamined Publication No. 5-328074. The pixel-clock frequency correction means adjusts the frequency of the pixel clock depending on the scan position of the laser beam to eliminate the distortion of a latent image caused by a variation in distance from the polygon mirror 10 to the surface of the photosensitive belt 11.
However, there are inevitably mounting errors of a laser scanner unit including a laser device, a polygon mirror and an image memory and errors caused by a variation in temperature or the like. Therefore, the distance between the laser scanner unit and the photosensitive belt 11 may vary color by color. This would result in a variation in the laser-beam scan speed at which the laser beam corresponding to each color scans over the photosensitive belt 11. More specifically, the distance between the polygon mirror 10 constituting each laser scanner unit and the photosensitive belt 11, which should be a reference distance D0, may be shifted to a distance D1 due to the mounting error of the polygon mirror 10 or some other factor. In this case, the scan distance of a laser beam over a specific time or a specific scan angle becomes a distance L1 different from a reference distance L0. This means that the latent image formed on the photosensitive belt 11 is shifted from the desired position. Since the distances between the laser scanner units for individual colors and the photosensitive belt 11 differ from each other, the amount of shift varies color by color. This may result in color deviation of a latent image formed on the photosensitive belt 11 by the laser beams of the individual colors.
Accordingly, it is an object of the present invention to provide an image forming apparatus and method, which can form an image on a photosensitive surface at a precise position.
It is another object of the present invention to provide an image forming apparatus and method, which prevent color deviation of an image formed on a photosensitive surface even when the distance between a laser scanner unit and the photosensitive body differs from the reference one.
According to the present invention, an image forming apparatus includes a timer for measuring a scan time or scan interval required for the laser beam to move from a first end position to a second end position on the photosensitive surface; and an adjuster or compensation circuit for adjusting the frequency of the timing signal depending on (as a function of) the scan time to keep a scan velocity of the laser beam with respect to the frequency of the timing signal at a predetermined constant value.
Since the scan velocity of the laser beam with respect to the frequency of the timing signal is kept constant by adjusting the frequency of the timing signal, an image is formed on the photosensitive surface at a precise position. Therefore, in the case of color image forming, color deviation of the image formed on the photosensitive surface can be eliminated.
The timer preferably operates according to a second timing signal whose frequency is higher than the frequency of the timing signal. Since the higher frequency is used to measure the scan time, the scan time can be measured more precisely.